Abstract:

A physical quantity sensor 1 comprises a driving circuit 4 that drives a
sensor element based on a reference signal; the sensor element 3 that is
driven by the driving circuit 4 to convert an externally applied physical
quantity to an electrical signal; and an amplifier circuit 5 that
amplifies an output signal of the sensor element 3. The driving circuit 4
controls a driving signal, which drives the sensor element, based on the
reference signal so that the driving signal is at a fixed level, and the
amplifier circuit 5 has a gain characteristic that amplifies the output
signal in a direction reverse to a direction in which the reference
signal varies or the driving signal of the driving circuit varies. This
configuration allows the physical quantity sensor to reduce output level
variations in the sensor output against signal level variations in the
reference signal and to make the sensitivity constant.

Claims:

1. A physical quantity sensor comprising:a driving circuit that drives a
sensor element based on a reference signal;said sensor element that is
driven by said driving circuit to convert an externally applied physical
quantity to an electrical signal; andan amplifier circuit that amplifies
an output signal of said sensor element whereinsaid driving circuit
controls a driving signal, which drives said sensor element, based on the
reference signal so that the driving signal is at a constant level,
andsaid amplifier circuit has a gain characteristic that amplifies an
output signal in a direction reverse to a direction in which the
reference signal varies or the driving signal of said driving circuit
varies.

2. The physical quantity sensor according to claim 1 whereinsaid amplifier
circuit has the gain characteristic that increases or decreases the
output signal in the reverse direction of an increase/decrease direction
of the reference signal or an increase/decrease direction of the driving
signal of said driving circuit determined by variations in the reference
signal, wherebyan output level of the output signal of said amplifier
circuit is kept constant regardless of variations in the reference
signal.

3. The physical quantity sensor according to claim 1 or 2 whereinsaid
amplifier circuit comprises an active circuit and a resistance circuit
having a plurality of resistor elements wherein a gain is determined by a
resistance ratio between the resistor elements andsaid resistance circuit
has a variable resistance circuit in at least a part of said resistor
elements, said variable resistance circuit making a resistance value
variable according to variations in the reference signal or variations in
the driving signal of said driving circuit.

4. The physical quantity sensor according to claim 1 or 2 whereinsaid
amplifier circuit comprises an inverting amplifier circuit or a
non-inverting amplifier circuit that has an operational amplifier and an
input resistance circuit and a feedback resistance circuit connected to
said operational amplifier wherein a gain is determined by a resistance
ratio between said input resistance circuit and said feedback resistance
circuit anda variable resistance circuit is formed in a resistor element
of at least one of said input resistance circuit and said feedback
resistance circuit, said variable resistance circuit making a resistance
value variable according to variations in the reference signal or
variations in the driving signal of said driving circuit.

5. The physical quantity sensor according to claim 3, further comprising a
first frequency converter that converts a level of the reference signal
or the driving signal of said driving circuit to a frequency whereinsaid
variable resistance circuit changes the resistance value by a pulse
modulated signal with a frequency obtained through the conversion by said
first frequency converter.

6. The physical quantity sensor according to claim 5, further comprising a
second frequency converter that converts a power supply voltage to a
frequency whereinsaid variable resistance circuit changes the resistance
value by a pulse modulated signal with a frequency, obtained through the
conversion by said second frequency converter, for making the gain of
said amplifier circuit proportional to an increase/decrease in the power
supply voltage.

7. The physical quantity sensor according to claim 1 or 2 whereinsaid
amplifier circuit is configured to determine a gain by a resistance ratio
between a plurality of resistor elements connected to an active
circuit,is configured to comprise a voltage-dividing circuit, which
divides a voltage of the reference signal or the driving signal of said
driving circuit into a predetermined steps, and comparison circuits and,
at the same time, to determine the gain by a resistance ratio between a
plurality of resistor elements, andhas a variable resistance circuit
formed in at least a part of said plurality of resistor elements for
making a resistance value variable,each of said comparison circuits
receives a divided voltage output of said voltage-dividing circuit at one
of input ends and receives a voltage, determined by a power supply
voltage, at another input end, andsaid variable resistance circuit
changes the resistance value by an output signal of each comparison
circuit to make the gain of said amplifier circuit inversely proportional
to an increase/decrease in the reference signal or the driving signal of
said driving circuit and, at the same time, to make the gain proportional
to an increase/decrease in the power supply voltage.

8. The physical quantity sensor according to claim 7, further comprising:a
resistance selection circuit that selects the resistance value according
to the output signal from said comparison circuits whereinsaid variable
resistance circuit changes the resistance value to a resistance value
selected by said resistance selection circuit.

9. The physical quantity sensor according to claim 3, further comprising a
voltage to current converter that converts a voltage of the reference
signal or a voltage of the driving signal of said driving circuit to a
current whereinsaid variable resistance circuit changes the resistance
value by the current obtained by the conversion through said voltage to
current converter.

10. The physical quantity sensor according to claim 1 or 2, further
comprising a power supply that supplies a power supply voltage to said
amplifier circuit whereinsaid amplifier circuit makes the gain
proportional to an increase/decrease in the power supply voltage.

11. The physical quantity sensor according to claim 1 or 2 whereinthe
output level of the output signal of said amplifier circuit is kept
constant against reference signal variations generated by a change in a
temperature.

Description:

FIELD OF THE INVENTION

[0001]The present invention relates to a physical quantity sensor, and
more particularly to the configuration of the output circuit of a
physical quantity sensor.

DESCRIPTION OF THE RELATED ART

[0002]Today, various types of physical quantity sensors are used.
Especially, many proposals are made for the correction of the sensor
output of an angular rate sensor typified by a vibratory gyroscope.

[0003]The prior-art technology disclosed in Patent Document 1 proposes a
method for changing the detection sensitivity of a physical quantity
sensor in proportion to a change in the power supply voltage at which the
physical quantity sensor operates. This method is known as ratiometric.
FIG. 17 is diagram showing the general configuration of a ratiometric
structure. In this ratiometric structure, a sensor 110 and an A/D
converter 120 receive the supply of the common power supply voltage Vdd.

[0004]If only the sensor 110 changes its output corresponding to
variations in the power supply voltage Vdd but the A/D converter 120 does
not correspond to variations in the power supply voltage Vdd, there is a
difference in the A/D-converted digital values. Conversely, if only the
A/D converter 120 changes its output corresponding to variations in the
power supply voltage Vdd but the output of the sensor 110 does not depend
on variations in the power supply voltage Vdd, there is also a difference
in the A/D-converted digital values.

[0005]On the other hand, if both the sensor 110 and the A/D converter 120
correspond to variations in the power supply voltage Vdd, there is no
difference in the A/D-converted digital values.

[0006]Patent Document 1 discloses an example of an angular rate sensor
that allows the detection sensitivity of a physical quantity sensor to be
adjusted when the power supply voltage varies. In particular, the sensor
in this example causes the detection sensitivity of a physical quantity
sensor to change in proportion to a change in the power supply voltage,
ensuring the ratiometric characteristic of the sensor sensitivity when
the base voltage of A/D conversion is decreased and thereby avoiding the
reading of an incorrect output level.

[0007]FIG. 18 is a diagram showing one example of the configuration of a
physical quantity sensor. Referring to FIG. 18, a physical quantity
sensor 101 comprises a sensor element 103 that outputs the output signal
according to an external force, a driving circuit 104 that generates a
driving signal that drives the sensor element 103, an amplifier circuit
107 that amplifies the output signal of the sensor element 103, and an
adjustment circuit 105 that adjusts the output signal based on the power
supply voltage Vdd to provide the ratiometric characteristic.

[0008]In the physical quantity sensor 101 described above, the driving
circuit 104 is driven by a voltage source 102. To stabilize the signal
level of the driving signal output by this driving circuit 104, a
reference signal generation circuit 109, independent of this voltage
source 102, is provided and the driving circuit 104 forms the driving
signal based on the reference voltage generated by this reference signal
generation circuit 109.

[0009]This physical quantity sensor 101 usually performs the operation
assuming that the output of the reference signal generation circuit 109
does not vary. This reference signal generation circuit 109 uses, for
example, a band gap type base voltage source (see Patent Document 1) as
the base voltage source to generate the signal. However, the inventor of
the present invention has confirmed that the signal level of the
reference signal generated by this base voltage source is not always
constant but is varied by various factors such as the temperature, power
supply voltage, and aging.

[0010]When the signal level of the reference signal generation circuit 109
varies, the output level of the sensor output also varies with the result
that the sensitivity is not constant but is varied.

[0011]To increase the detection accuracy of the physical quantity sensor,
there is a need for a physical quantity sensor that does not vary the
output level of the sensor output but gives a constant sensitivity even
when the signal level of the reference signal generation circuit 109
varies.

[0012]Therefore, it is an object of the present invention to solve the
conventional problem and to provide a physical quantity sensor that
decreases the output level variations in the sensor output when the
signal level of the reference signal varies for keeping the sensitivity
constant.

[0013]A physical quantity sensor according to the present invention, which
amplifies the signal of the sensor output of the physical quantity
sensor, adjusts the gain to reduce variations in the output level when
the reference signal varies. In adjusting the gain, the direction of the
gain characteristic of sensor output signal amplification is made reverse
to the direction of the variation characteristic of the sensor output
when the reference signal varies. Adjusting the gain in this way cancels
the variations in the sensor output caused by variations in the reference
signal.

[0014]The physical quantity sensor of the present invention comprises a
driving circuit that drives a sensor element based on a reference signal;
the sensor element that is driven by the driving circuit to convert an
externally applied physical quantity to an electrical signal; and an
amplifier circuit that amplifies the output signal of the sensor element.
The driving circuit controls the driving signal, which drives the sensor
element, based on the reference signal so that the driving signal is at a
fixed level. The amplifier circuit of the present invention has a gain
characteristic that amplifies an output signal in a direction reverse to
a direction in which the reference signal varies or the driving signal of
the driving circuit varies.

[0015]It is an object of the present invention to reduce variations in the
output level of a sensor output when the signal level of the reference
signal varies. Because the driving circuit forms the driving signal based
on the reference signal, the driving signal of the driving circuit varies
according to the reference signal. To address this problem, the present
invention provides the amplifier circuit with a gain characteristic which
amplifies the output signal of the amplifier circuit in a direction
reverse to the direction in which the reference signal varies. This
configuration reduces the output level variations in the sensor output
generated when the signal level of the reference signal varies.

[0016]It is also possible for the amplifier circuit of the present
invention to have a gain characteristic that not only changes the gain
according to the variations in the reference signal but also amplifies
the output signal of the amplifier circuit in a direction reverse to the
direction of the variations in the driving signal that varies according
to the reference signal.

[0017]The gain of the amplifier circuit has a characteristic that
amplifies the output signal in the reverse direction of the variations in
the reference signal or the variations in the driving signal of the
driving circuit. For example, this gain characteristic is a
characteristic that increases or decreases the output signal in the
reverse direction of an increase/decrease direction of the reference
signal or an increase/decrease direction of the driving signal of the
driving circuit determined by variations in the reference signal, whereby
an output level of the output signal of the amplifier circuit is kept
constant regardless of variations in the reference signal.

[0018]When the output signal varies due to variations in the reference
signal or variations in the driving signal of the driving circuit, the
amplifier circuit amplifies the signal in the direction in which the
variations in the output signal are canceled. Therefore, this
configuration reduces the effect of the variations in the reference
signal, or the effect of the variations in the driving signal of the
driving circuit, on the output signal output from the amplifier circuit.

[0019]The amplifier circuit of the physical quantity sensor of the present
invention comprises an active circuit and a resistance circuit having a
plurality of resistor elements wherein the gain is determined by a
resistance ratio between the resistor elements. The resistance circuit
has a variable resistance circuit in at least a part of the resistor
elements, wherein the variable resistance circuit makes the resistance
value variable according to variations in the reference signal or
variations in the driving signal of the driving circuit.

[0020]The variable resistance circuit changes the resistance values of
resistor elements to change the resistance ratio and thereby change the
gain of the amplifier circuit. Changing the resistance value of the
variable resistance circuit according to the variations in the reference
signal or the variations in the driving signal of the driving circuit
changes the gain of the amplifier circuit according to the variations in
the reference signal or the variations in the driving signal of the
driving circuit.

[0021]The configuration for making the resistance value of this variable
resistance circuit variable can be implemented in one of multiple
embodiments.

[0022]In a first embodiment, the voltage of the reference signal or the
driving signal is converted to a frequency which is used to change the
resistance value. In a second embodiment, the resistance value is
selected and switched according to the voltage of the reference signal or
the driving signal. In a third embodiment, the voltage of the reference
signal or the driving signal is converted to a current which is used to
change the resistance value.

[0023]In the embodiments, the gain of the amplifier circuit is made
proportional to an increase/decrease in the power supply voltage to
provide the physical quantity sensor with the ratiometric characteristic.

[0024]The physical quantity sensor in the first embodiment of the present
invention converts the voltage of the reference signal or the driving
signal to a frequency which is used to change the resistance value. The
physical quantity sensor comprises a first frequency converter that
converts the level of the reference signal or the driving signal of the
driving circuit to a frequency and an amplifier circuit that determines
the gain by the resistance ratio between the multiple resistor elements
connected to the active circuit.

[0025]In this amplifier circuit, a variable resistance circuit that makes
the resistance value variable by the pulse modulated signal is formed in
at least a part of the multiple resistor elements connected to the active
circuit. The resistance value of this variable resistance circuit is
changed by the pulse modulated signal with a frequency obtained through
the conversion by the first frequency converter to make the gain of the
amplifier circuit inversely proportional to an increase/decrease in the
reference signal or the driving signal of the driving circuit.

[0026]In addition, the physical quantity sensor of the present invention
can have the ratiometric characteristic. The ratiometric characteristic
can be achieved by making the gain of the amplifier circuit, which
outputs the output signal, proportional to an increase/decrease in the
power supply voltage.

[0027]To provide the ratiometric characteristic, the physical quantity
sensor of the present invention further comprises a second frequency
converter that converts the power supply voltage to a frequency. In the
amplifier circuit, a variable resistance circuit that makes the
resistance value variable by the pulse modulated signal is formed in at
least a part of the multiple resistor elements connected to the active
circuit. The resistance value of the variable resistance circuit is
changed by the pulse modulated signal with a frequency, obtained through
the conversion by the second frequency converter, to make the gain of the
amplifier circuit proportional to an increase/decrease in the power
supply voltage.

[0028]Therefore, the physical quantity sensor of the present invention can
output an output signal that is not affected by the variations in the
reference signal and that has the ratiometric characteristic. Both
variability resistance against the reference signal and the ratiometric
characteristic can be achieved by adjusting the gain of the amplifier
circuit. For achieving variability resistance against the reference
signal, the gain is adjusted so that the gain is increased or decreased
in the reverse direction of an increase/decrease in the reference signal
or the driving signal that is dependent on the reference signal. For
achieving the ratiometric characteristic, the gain is adjusted so that
the gain is proportional to an increase/decrease in the power supply
voltage.

[0029]To adjust the gain in both cases, a variable resistance circuit is
formed in at least a part of the resistor elements, connected to the
active circuit of the amplifier circuit, for making the resistance value
variable by the pulse modulated signal. The resistance value of this
variable resistance circuit is made variable based on the pulse modulated
signal generated by converting the reference signal, driving signal, or
power supply voltage via the frequency converter.

[0030]In a second embodiment of the physical quantity sensor of the
present invention, the resistance value is selected and switched
according to the voltage of the reference signal and the driving signal.
The amplifier circuit is configured to determine the gain by the
resistance ratio between a plurality of resistor elements connected to an
active circuit, and comprises a voltage-dividing circuit, which divides
the voltage of the reference signal or the driving signal of the driving
circuit into a predetermined steps, and comparison circuits. This
configuration also has both variability resistance against the reference
signal and the ratiometric characteristic described above.

[0031]In the second embodiment, a variable resistance circuit is formed in
at least a part of the plurality of resistor elements for making the
resistance value variable. Each of the comparison circuits receives a
divided voltage output of the voltage-dividing circuit at one of input
ends and receives a voltage, determined by the power supply voltage, at
another input end. The variable resistance circuit changes the resistance
value by an output signal of each comparison circuit to make the gain of
the amplifier circuit inversely proportional to an increase/decrease in
the reference signal or the driving signal of the driving circuit and, at
the same time, to make the gain proportional to an increase/decrease in
the power supply voltage.

[0032]This configuration, in which the circuit configuration comprising
the voltage-dividing circuit and the comparison circuits is included,
allows two types of adjustment to be made: one is to make the gain of the
amplifier circuit inversely proportional to an increase/decrease in the
reference signal and the driving signal of the driving circuit and the
other is to make the gain of the amplifier circuit proportional to an
increase/decrease in the power supply voltage.

[0033]The amplifier circuit of the physical quantity sensor of the present
invention comprises an inverting amplifier circuit or a non-inverting
amplifier circuit that has an operational amplifier and an input
resistance circuit and a feedback resistance circuit connected to the
operational amplifier wherein the gain is determined by a resistance
ratio between the input resistance circuit and the feedback resistance
circuit. A variable resistance circuit, which makes the resistance value
variable according to variations in the reference signal or variations in
the driving signal of the driving circuit, is formed in a resistor
element of at least one of the input resistance circuit and the feedback
resistance circuit. The resistance value of the variable resistance
circuit is made variable to change the resistance ratio between the input
resistance circuit and the feedback resistance circuit for adjusting the
gain.

[0034]In a third embodiment of the physical quantity sensor of the present
invention, the voltage of the reference signal or the driving signal is
changed to a current which is used to change the resistance value. The
amplifier circuit has a voltage/current converter that converts the
voltage of the reference signal or the voltage of the driving signal of
the driving circuit to a current. The variable resistance circuit changes
the resistance value by the current obtained by the conversion by this
voltage to current converter.

[0035]The physical quantity sensor of the present invention can make the
output level of the output signal of the amplifier circuit constant
against variations in the reference signal. The factors of variations in
the reference signal include variations in the reference signal due to a
change in the temperature, variations in the power supply voltage
supplied to the reference signal formation circuit that forms the
reference signal, and variations in the output signal levels caused by
the aging of the reference signal formation circuit. The physical
quantity sensor of the present invention is compatible with any of those
factors.

[0036]The physical quantity sensor of the present invention decreases the
output level variations in the sensor output when the signal level of the
reference signal varies, and keeps the output sensitivity constant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0037]FIG. 1 is a general block diagram showing the general configuration
of a physical quantity sensor of the present invention.

[0038]FIG. 2 is a general block diagram showing the general configuration
of a physical quantity sensor of the present invention.

[0039]FIG. 3 is a diagram showing examples of the configuration of an
amplifier circuit provided in the physical quantity sensor of the present
invention.

[0040]FIG. 4 is a diagram showing examples of the configuration of a
conversion circuit provided in the physical quantity sensor of the
present invention.

[0041]FIG. 5 is a diagram showing the general configuration of a physical
quantity sensor in a first embodiment of the present invention that
converts the resistance value via voltage to frequency conversion.

[0042]FIG. 6 is a diagram showing the general operation for canceling and
decreasing the variations in the reference signal using the reverse
characteristic of the gain of the amplifier circuit.

[0043]FIG. 7 is a diagram showing an example of the general configuration
of the physical quantity sensor in the first embodiment of the present
invention in which both variability resistance against the reference
signal and ratiometric characteristic are provided.

[0044]FIG. 8 is a diagram showing an example of the general configuration
of the physical quantity sensor in the first embodiment of the present
invention in which both variability resistance against the reference
signal and ratiometric characteristic are provided.

[0045]FIG. 9 is a diagram showing the configuration of the physical
quantity sensor in the first embodiment of the present invention.

[0046]FIG. 10 is a diagram showing the relation between the power supply
voltage and the midpoint voltage.

[0047]FIG. 11 is diagram showing examples of the configuration, in which
the output level variations in the sensor output are decreased when the
signal level of the reference signal varies, in the physical quantity
sensor in the first embodiment of the present invention.

[0048]FIG. 12 is a diagram showing examples of the configuration of the
present invention in which the output level variations in the sensor
output are decreased when the signal level of the reference signal
varies.

[0049]FIG. 13 is a diagram showing examples of the configuration of the
physical quantity sensor in the first embodiment of the present invention
in which the output level variations in the sensor output are decreased
when the signal level of the reference signal varies or the power supply
voltage varies.

[0050]FIG. 14 is a diagram showing a physical quantity sensor in a second
embodiment of the present invention in which the variable resistance
circuit of the feedback resistor of the operational amplifier (OP Amp) is
made variable by comparing the reference signal and the power supply
voltage.

[0051]FIG. 15 is a diagram showing the relation between the direction in
which the reference signal increases or decreases and the direction in
which the gain selected by the switch is increased or decreased.

[0052]FIG. 16 is a diagram showing a physical quantity sensor in a third
embodiment of the present invention that changes the resistance value by
converting the voltage of the reference signal or the driving signal to a
current.

[0053]FIG. 17 is diagram showing the general configuration of a
ratiometric structure.

[0054]FIG. 18 is a diagram showing an example of the configuration of a
physical quantity sensor.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0055]A physical quantity sensor of the present invention will be
described in detail below with reference to the drawings.

[0056]The general configuration of the physical quantity sensor of the
present invention will be described with reference to FIGS. 1 and 2, and
the general configuration of an amplifier circuit provided in the
physical quantity sensor of the present invention will be described with
reference to FIGS. 3 and 4.

[0057]A first embodiment in which the voltage of the reference signal or
the driving signal is converted to a frequency for changing the
resistance value will be described with reference to FIG. 5 to FIG. 13, a
second embodiment in which the resistance value is selected and switched
according to the voltage of the reference signal or the driving signal
will be described with reference to FIG. 14 and FIG. 15, and a third
embodiment in which the voltage of the reference signal or the driving
signal is converted to a current for changing the resistance value will
be described with reference to FIG. 16.

[0058]FIG. 9 to FIG. 12 are diagrams showing the detailed configuration of
the physical quantity sensor of the present invention that decreases the
output level variations in the sensor output when the signal level of the
reference signal varies. FIG. 13 to FIG. 15 show examples of the physical
quantity sensor of the present invention that includes two
configurations: one is a configuration for stabilizing the output signal
level when the reference signal varies, and the other is a configuration
for providing the ratiometric characteristic.

[0059]FIGS. 1 and 2 are general block diagrams showing the general
configuration of the physical quantity sensor of the present invention.
FIG. 1 is a diagram showing an example of the configuration of the
physical quantity sensor of the present invention that decreases the
output level variations in the sensor output when the signal level of the
reference signal varies. FIG. 2 is a diagram showing an example of the
configuration of the physical quantity sensor of the present invention
that has variability resistance against the reference signal for
suppressing the output level variations in the sensor output when the
signal level of the reference signal varies, as well as the ratiometric
characteristic.

[0060]Referring to FIGS. 1 and 2, a physical quantity sensor 1 comprises a
reference signal generation circuit 9 that generates the reference signal
(Vref(α)), a driving circuit 4 that generates the driving signal
based on the reference signal (Vref(α)), a sensor element 3 that is
driven by the driving circuit 4 to convert an externally applied physical
quantity to an electrical signal, and an amplifier circuit 5 that
amplifies the output signal of the sensor element 3, changes the gain
based on the reference signal (Vref(α)), and outputs the output
signal according to variations in the reference signal (Vref(α)).

[0061]The driving circuit 4 controls the driving signal, which is supplied
to the sensor element 3, so that the driving signal level becomes
constant based on the reference signal (Vref(α)) formed
independently of a voltage source 2. In FIGS. 1 and 2, the driving signal
is indicated by aVref(α). Here, "a" is a coefficient representing
the relation between the driving signal and the reference signal
(Vref(α)) in the driving circuit 4. The reference signal
(Vref(α)) varies according to temperature variations, power supply
voltage variations, or the aging of the reference signal formation means
that forms the reference signal. So, the reference signal can be
represented as the reference signal Vref(α) where "a" is a
variation parameter. In the description below, the reference signal Vref
is represented as Vref(α).

[0062]The sensor element 3, which is driven by the driving signal
aVref(α) received from the driving circuit 4, outputs the output
signal corresponding to an external force. Here, the output signal from
the sensor element 3 can be represented as SaVref(α) where S is the
contribution factor of the amplitude of the output signal from the sensor
element 3 that is generated by the external force.

[0063]The amplifier circuit 5 amplifies the signal (SaVref(α))
received from the sensor element 3 and outputs the amplified signal. This
amplifier circuit 5 outputs the output signal ASaVref(α) generated
by multiplying the received SaVref(α) by A where "A" is the gain.

[0064]Here, the signal level of the output signal can be made constant
regardless of the variations in the reference signal Vref(α) by
changing the gain A based on the variations in the reference signal
Vref(α) that varies according to the variation parameter "α".

[0065]To do so, the gain A of the amplifier circuit 5 is changed so that
the amplifier circuit 5 has the reverse characteristic of the variation
characteristic of the reference signal Vref(α). This makes the
output signal level constant regardless of the variations in the
reference signal Vref(α).

[0066]In this case, by changing the gain A of the amplifier circuit 5 so
that amplifier circuit has the reverse characteristic of the variation
characteristic of the reference signal Vref(α), it is possible to
cancel those variations and to remove the variation characteristic of the
reference signal Vref(α) from the output signal even when the
reference signal varies and, as a result, the output signal of the sensor
element 3 varies.

[0067]When the opposite characteristic is represented as
Vref-1(α), the gain A of the amplifier circuit 5 is
represented as A0Vref-1(α). Here, the relation between
Vref(α) and Vref-1(α) can be represented as
Vref(α) Vref-1(α)=1 and, therefore, the output signal
ASaVref(α) of the amplifier circuit 5 can be represented as
follows.

The above expression indicates that the output signal ASaVref(α) is
independent of the reference signal Vref(α). In the above
description, A0 is the gain that is set.

[0068]Referring to FIG. 2, the amplifier circuit 5 receives the power
supply voltage Vdd and adjusts the gain so that the gain is proportional
to an increase/decrease in the power supply voltage Vdd. This adjustment
allows the output signal to have the ratiometric characteristic. This
configuration allows the detection sensitivity of the physical quantity
sensor to change in proportion to a change in the power supply voltage.

[0069]The following describes examples of the configuration of the
amplifier circuit provided in the physical quantity sensor of the present
invention with reference to FIG. 3 and FIG. 4.

[0070]Referring to FIGS. 3A and 3B, the amplifier circuit 5 has a signal
amplification unit 7 that can increase or decrease the gain. The signal
amplification unit 7, which comprises an active circuit 71 and a
resistance circuit 72 having multiple resistor elements, determines the
gain according to the resistance ratio of the resistor elements. At least
one of the resistor elements in the resistance circuit 72 is a variable
resistance circuit 73. This variable resistance circuit 73 changes the
resistance value according to the reference signal or the driving signal.

[0071]The signal amplification unit 7 performs the amplifier circuit
operation via the resistance circuit 72 and the active circuit 71 to
change the resistance value of the variable resistance circuit 73 in the
resistance circuit 72 and thereby to increase or decrease the gain.
Changing the resistance value of this variable resistance circuit 73
according to the reference signal or the driving signal can increase or
decrease the gain according to the reference signal or the driving
signal. The resistance value of the variable resistance circuit 73 is
increased or decreased based on the parameter generated by a conversion
unit 6 by converting the voltage of the reference signal or the driving
signal. FIG. 3B shows the configuration in which the resistance value of
the variable resistance circuit 73 is increased or decreased in
proportion to an increase/decreased in the power supply voltage.

[0072]This configuration provides variability resistance against the
reference signal for decreasing the output level variations in the sensor
output against the variations in the signal level of the reference
signal, as well as the ratiometric characteristic.

[0073]Referring to FIG. 3C, the signal amplification unit 7 can be
constructed as an inverting amplifier circuit or a non-inverting
amplifier circuit in which the active circuit 71 is used as an
operational amplifier, the variable resistance circuit 73 includes an
input resistance circuit and a feedback resistance circuit, and the input
resistance circuit and the feedback resistance circuit are connected to
the operational amplifier. The gain of the signal amplification unit 7 is
defined by the resistance value ratio between the input resistance
circuit and the feedback resistance circuit. A resistor element included
in at least one of the input resistance circuit and the feedback
resistance circuit forms a variable resistance circuit that makes the
resistance value variable when the reference signal varies or the driving
signal output from the driving circuit varies.

[0074]FIG. 4 is a diagram showing the configurations of the conversion
circuit. FIG. 4A to FIG. 4C show the configurations in which the voltage
of the reference signal or the driving signal is converted to the
frequency, resistance, and current parameters.

[0075]In the configuration shown in FIG. 4A, a frequency conversion unit
61 is used as the conversion unit 6 to convert the voltage of the
reference signal or the driving signal to a frequency parameter for
increasing or decreasing the resistance value of the variable resistance
circuit 73 using the frequency. In the configuration shown in FIG. 4B, a
voltage to resistance conversion unit 62 is used as the conversion unit 6
to convert the voltage of the reference signal or the driving signal to a
resistance parameter for increasing or decreasing the resistance value of
the variable resistance circuit 73 using the resistance. In the
configuration shown in FIG. 4C, a voltage to current conversion unit 63
is used as the conversion unit 6 to convert the voltage of the reference
signal or the driving signal to a current parameter for increasing or
decreasing the resistance value of the variable resistance circuit 73
using the current.

[0076]The following describes the operation of the configuration shown in
FIG. 4A with reference to FIG. 5 to FIG. 8. FIG. 5 is a diagram showing
an example of the configuration of the physical quantity sensor of the
present invention that decreases the variations in the output level of
the sensor output when the signal level of the reference signal varies.
On the other hand, FIGS. 7 and 8 are diagrams showing examples of the
configuration of the physical quantity sensor of the present invention
that provides variability resistance against the reference signal for
decreasing the output level variations in the sensor output when the
signal level of the reference signal varies, as well as the ratiometric
characteristic.

[0077]The example of the configuration shown in FIG. 5 is the example of
the configuration shown in FIG. 1 wherein the frequency conversion unit
61 and the signal amplification unit 7 constitute the amplifier circuit
5. Because FIG. 5 is different from FIG. 1 only in the configuration of
the amplifier circuit 5, only the configuration of the amplifier circuit
5 is described below and the description of the common part is omitted.

[0078]The driving circuit 4 controls the driving signal of the sensor
element 3, based on the reference signal (Vref(α)) formed by the
reference signal generation circuit 9 independently of the voltage source
2, so that the level of the driving signal is at a fixed level.

[0079]The sensor element 3, which is driven by the driving signal
aVref(α) received from the driving circuit 4, outputs the output
signal SaVref(α) corresponding to an external force. "a" is a
coefficient representing the relation between the driving signal and the
reference signal, and "S" is the contribution factor of the amplitude of
the output signal of the sensor element 3 generated by the external
force.

[0080]The frequency conversion unit 61 included in the amplifier circuit 5
receives the voltage of the output signal SaVref(α) of the sensor
element 3 and the voltage of the reference signal Vref(α) or the
driving signal aVref(α) and converts the received voltage to a
frequency.

[0081]The frequency conversion unit 61 changes the resistance ratio of the
variable resistance circuit to change the gain "A", multiplies the
received SaVref(α) by A, and outputs the output signal
ASaVref(α).

[0082]By adjusting the gain A based on the variations in the reference
signal Vref(α) that varies according to the variation parameter "",
the signal level of the output signal is kept constant regardless of the
variations in the reference signal Vref(α). To do so, the signal
amplification unit 7 adjusts the gain A so that it provides the reverse
characteristic of the variation characteristic of the reference signal
Vref(α), thus making the signal level of the output signal constant
regardless of the variations in the reference signal Vref(α).

[0083]FIG. 6 is a general diagram showing the operation for canceling and
decreasing the variations in the reference signal using the reverse
characteristic of the gain of the amplifier circuit.

[0084]FIGS. 6A-6C show a case in which the gain of the amplifier circuit
does not change when the reference signal varies. On the other hand,
FIGS. 6D-6F show a case in which the amplifier circuit has the gain of
the reverse characteristic. FIG. 6A and FIG. 6D show the characteristic
of the reference signal Vref(α) for the variation parameter
α, FIG. 6B and FIG. 6E show the gain for the variation parameter
α, and FIG. 6C and FIG. 6F show the output signal for the variation
parameter α.

[0085]In the case where the gain of the amplifier circuit does not change
when the reference signal varies (FIG. 6B), the variations in the
reference signal Vref(α) are reflected on the output signal. So,
when the reference signal Vref(α) varies according to the variation
parameter α (FIG. 6A), the output signal varies according to the
variations in the reference signal Vref(α) (FIG. 6C).

[0086]On the other hand, in the case where the gain of the amplifier
circuit is changed when the reference signal varies (FIG. 6E), the
variations in the reference signal Vref(α) are canceled by the
variations in the gain (FIG. 6F). So, even when the reference signal
Vref(α) varies according to the variation parameter α, the
output signal remains constant regardless of the variations in the
reference signal Vref(α) (FIG. 6F). For example, when the reference
signal Vref(α) is represented as (aα+b) for the variation
parameter α (FIG. 6D), the gain A is changed in inverse proportion
to the variations in the reference signal Vref(α) (for example,
A0/(aα+b)). As a result, the output signal is amplified with
the gain A0 regardless of the variations in the reference signal
Vref(α) and therefore the effect of the variation parameter α
is removed.

[0087]To change the gain A of the signal amplification unit 7 according to
the reference signal Vref(α) or the driving signal aVref(α),
the amplifier circuit 5 uses the frequency conversion unit 61 to convert
the reference signal Vref(α) or the driving signal aVref(α)
to the frequency signal. The signal amplification unit 7 uses this
frequency signal, obtained by the conversion of the frequency conversion
unit 61, to change the resistance value of the variable resistance
circuit connected to the active circuit and, by doing so, changes the
gain. The circuit configuration for changing the gain using this
frequency signal will be described later with reference to the circuit
examples in FIGS. 11 and 12.

[0088]FIGS. 7 and 8 show examples of the configuration of the physical
quantity sensor of the present invention that has the configuration in
which, as in FIG. 5, the output level variations in the sensor output are
decreased when the signal level of the reference signal varies and, in
addition, the configuration in which the ratiometric characteristic is
provided to make the gain of the amplifier circuit variable in proportion
to the variations in the power supply voltage. The configuration already
described in FIG. 5 will be omitted in the description below.

[0089]In FIGS. 7 and 8, the physical quantity sensor 1 has the same
configuration as that shown in FIG. 5 and comprises the sensor element 3,
driving circuit 4, amplifier circuit 5, and reference signal generation
circuit 9. The amplifier circuit 5 has the following two configurations.
In one configuration, the signal is amplified by again having the
characteristic reverse to the variations in the reference signal
Vref(α) to decrease the output level variations in the sensor
output when the signal level of the reference signal varies. In the other
configuration, the signal is amplified by a gain having the
characteristic in the same direction as that of the variations in the
power supply voltage Vdd to decrease the output level variations in the
sensor output when the power supply voltage varies.

[0090]In the configuration shown in FIG. 7, the amplifier circuit 5
changes the gain A based on the reverse characteristic of the reference
signal Vref(α) or the driving signal aVref(α). For example,
in addition to a first signal amplification unit 7A, the amplifier
circuit 5 has a first frequency conversion unit 6A that converts the
reference signal Vref(α) or the driving signal aVref(α) to a
frequency signal. The first signal amplification unit 7A uses the
frequency signal, obtained by the conversion of the first frequency
conversion unit 6A, to change the resistance value of the variable
resistance circuit connected to the active circuit and thereby changes
the gain. In addition, the first frequency conversion unit 6A also
receives the power supply voltage Vdd, and the first signal amplification
unit 7A uses the frequency signal, converted by the first frequency
conversion unit 6A, to change the resistance value of the variable
resistance circuit connected to the active circuit and thereby changes
the gain in proportion to the power supply voltage Vdd.

[0091]In the configuration shown in FIG. 8, the amplifier circuit 5
changes the gain A of the amplifier circuit based on the same
increase/decrease characteristic as that of the power supply voltage Vdd.
For example, the amplifier circuit 5 has a second signal amplification
unit 7B and a second frequency conversion unit 6B that converts the power
supply voltage Vdd to a frequency signal. The second signal amplification
unit 7B uses the frequency signal, obtained by the conversion of the
second frequency conversion unit 6B, to change the resistance value of
the variable resistance circuit connected to the active circuit and
thereby changes the gain in proportion to the power supply voltage Vdd.

[0092]In addition, the first signal amplification unit 7A and the second
signal amplification unit 7B can be combined into one amplifier circuit.
An example of the configuration of this amplifier circuit, which has the
two functions described above, will be described later with reference to
FIG. 13 to FIG. 16. The two functions are the function to change the gain
based on the variations in the reference signal or the driving signal and
the function to change the gain based on the variations in the power
supply voltage.

[0093]Next, with reference to FIG. 9 to FIG. 12, the following describes
an example of the detailed configuration of the physical quantity sensor
of the present invention that decreases the variations in the output
level of the sensor output when the signal level of the reference signal
varies.

[0094]FIG. 9 is a block diagram showing an example of the configuration of
the physical quantity sensor of the present invention, and this figure
shows the configuration in FIG. 5 more in detail. Because the general
configuration of the sensor element 3, driving circuit 4, amplifier
circuit 5, and reference signal generation circuit 9 is already described
in FIG. 5, the description is omitted here and an example of
configuration of each circuit will be described.

[0095]The sensor element 3, which may be configured by a piezoelectric
vibrator such as a crystal resonator, comprises a driving unit 3A that
excites and vibrates the piezoelectric vibrator and a detection unit 3B
whose vibration state is changed by an externally-applied force. For
example, when the piezoelectric vibrator is configured by a
tuning-fork-type piezoelectric vibrator that has multiple legs, the
driving unit 3A comprises a driving leg and a driving electrode provided
on the driving leg and is excited by the driving signal, supplied from
the driving circuit 4, for oscillation and vibration.

[0096]On the other hand, the detection unit 3B comprises a detection leg
and a detection electrode provided on the detection leg. For example, the
vibration state of the detection leg is changed by the Coriolis force
generated by an externally-applied force, and the detection electrode
detects this vibration state as the detection signal. The detection unit
3B outputs the detection signal to the amplifier circuit 5 via a
detection circuit 8.

[0097]The driving circuit 4 is a circuit that forms the driving signal for
exciting and driving the driving unit 3A of the sensor element 3. The
driving circuit 4 feeds back the signal from the driving unit 3A to
adjust the phase and the amplitude for forming a driving signal at a
predetermined frequency. FIG. 9 shows an example of the constant current
control in which the current value of the driving signal is controlled at
a fixed value.

[0098]A current detection circuit 4A detects the current of a signal
detected at one of the electrodes of the driving unit 3A, and an
Automatic Gain Control circuit (AGC circuit) 4B forms a control signal
that makes the current value of this detected signal constant and adjusts
the gain of a gain-variable amplifier circuit 4C using this control
signal. For example, the Automatic Gain Control circuit (AGC circuit) 4B
comprises an effective value circuit 4Ba that calculates the effective
value of the output signal of the current detection circuit 4A and a
comparison circuit 4Bb that calculates the difference between the output
of the effective value circuit 4Ba and the reference signal Vref and
compares the difference with a setting value. In this configuration, the
current value of the driving signal is compared with the reference
signal, and the gain of the gain-variable amplifier circuit 4C is
adjusted so that the current value of the driving signal becomes constant
with the reference signal as the base signal.

[0099]The reference signal (Vref(α)) can be generated by a midpoint
voltage generating unit 9B and a reference voltage generating unit 9A.
The midpoint voltage generating unit 9B and the reference voltage
generating unit 9A receive the voltage from the voltage source 2. For
example, the midpoint voltage generating unit 9B generates Vm from the
midpoint between the voltage Vdd of the voltage source 2 and Vss. When
Vss is at the ground voltage, the midpoint voltage Vm is Vdd/2. FIG. 10
shows this voltage relation.

[0100]The reference voltage generating unit 9A uses the midpoint voltage
Vm, generated by the midpoint voltage generating unit 9B, to generate a
voltage independent of the power supply voltage Vdd. The problem is that,
the reference voltage generating unit 9A generates the reference signal
as the base signal for making the current of the driving signal formed by
the driving circuit 4 constant, and the signal level of the reference
signal varies according to the temperature, power supply voltage, or
aging in the actual circuit configuration. Those variations in the signal
level of the reference signal vary the signal level of the driving signal
and become a factor that varies the signal level of the output signal
that is output via the sensor element 3 and the signal amplification unit
7. The present invention decreases the variations in the output signal
generated by the variations in the reference signal that is used as the
base signal.

[0101]In the signal amplification unit 7 included in the amplifier circuit
5 of the present invention for making the gain variable, at least one of
multiple resistor elements connected to the active circuit of the
amplifier circuit is configured by a variable resistance circuit, and the
resistance value of this variable resistance circuit is made variable for
changing the gain of the amplifier circuit.

[0102]Referring to the circuit configuration shown in FIG. 11 and FIG. 12,
the following describes the detailed configuration for decreasing the
output level variations in the sensor output when the signal level of the
reference signal varies.

[0103]FIG. 11A shows an example of an inverting amplifier circuit, and
FIG. 11B shows an example of a non-inverting amplifier circuit. Although
the signs are reversed, the magnitude of the gain of an inverting
amplifier circuit and a non-inverting amplifier circuit is determined by
the input resistance Rs and the feedback resistance Rf connected to the
operational amplifier (OP Amp). The following description is based
primarily on the example of the inverting amplifier circuit in FIG. 11A.

[0104]In FIG. 11A, the amplifier circuit 5 comprises a frequency converter
(linear VCO) 6a which converts the voltage of the reference signal Vref
to a frequency and the signal amplification unit 7 in which an input
resistor 11 and a feedback resistor 10 of the operational amplifier (OP
amp) are connected.

[0105]Because the gain of the signal amplification unit 7 is defined as
(-Rf/RS), forming the feedback resistor 10 with a variable resistance
circuit 10a and changing the resistance value of this variable resistance
circuit 10a using the reference signal Vref, which has been converted to
a frequency signal by the frequency conversion unit (linear VCO) 6a,
allow the increase/decrease direction of the gain to be adjusted in the
opposite direction of the variation direction of the reference signal
Vref. For example, when the reference signal Vref is increased, the
resistance value of the variable resistance circuit 10a is decreased to
decrease the gain of the signal amplification unit 7; conversely, when
the reference signal Vref is decreased, the resistance value of the
variable resistance circuit 10a is increased to increase the gain of the
signal amplification unit 7.

[0106]The relation among the reference signal, the frequency signal, the
resistance value of the variable resistance circuit, and the gain is as
follows. That is, the reference signal and the frequency signal have the
forward increase characteristic, the frequency signal and the resistance
value have the reverse increase characteristic, and the resistance value
and the gain have the same increase characteristic. This means that the
reference signal and the gain have the reverse increase characteristic,
meaning that the gain is decreased when the reference signal is increased
and that the gain is increased when the reference signal is decreased.
Therefore, the gain of the amplifier circuit has the reverse
characteristic of the reference signal.

[0107]To increase or decrease the resistance value of this variable
resistance circuit, the so-called switched capacitor circuit, which
transfers charges by switching the capacitor connection state, is
provided in the feedback stage. The gain of this switched capacitor
circuit is made variable based on the pulse modulated signal.

[0108]The amplification circuit 7 shown in FIG. 11A has the configuration
of an inverting amplifier circuit having an operational amplifier (OP
Amp) 21. A switched capacitor circuit and a filter capacitor 7c, which
are connected in parallel to form the variable resistance circuit 10a,
are connected between the output end and the input end (inverting input
terminal) of the operational amplifier (OP Amp) 21 as the feedback
resistor, and the input resistor 11 is connected to the inverting input
terminal of the operational amplifier (OP Amp) 21. The switched capacitor
circuit is configured by a switch 7a, which has two contacts, and a
capacitor 7b.

[0109]The switch 7a can be configured by a transfer gate (transmission
gate) implemented by a MOS element, and the contact state of the switch
7a is configured in such away that the contact state is switched
according to the frequency signal from the frequency converter 6a. That
is, the connection state of the capacitor is switched according to the
frequency signal. The switch 7a can be fabricated in the semiconductor
process in the same way as the capacitors 7b and 7c and the input
resistor 11, and can be configured on the same semiconductor chip. This
configuration allows the elements to have the same temperature
characteristic.

[0110]One end of the capacitor 7b is connected to the midpoint voltage Vm,
and the other end is connected to a fixed contact of the switch 7a. The
switch 7a, as well as filter capacitor 7c, is connected between the
inverting input terminal and the output terminal of the operational
amplifier (OP Amp) 21. The non-inverting input terminal of the
operational amplifier (OP Amp) 21 is connected to the mid-point voltage
Vm.

[0111]The switched capacitor circuit comprises the switch 7a and the
capacitor 7b. When the contact of the switch 7a is connected to the
inverting input terminal side, the capacitor 7b stores the voltage of the
detection output; next, when the switch 7a is connected to the output
terminal side, the charge stored in the capacitor 7b is discharged.

[0112]As described above, the connection state of the capacitor 7b is
switched by switching the switch 7a between the inverting input terminal
side and the signal terminal side according to the frequency signal of
the frequency converter 6a.

[0113]The high-speed switching operation of the switch 7a described above
causes the switched capacitor circuit to perform the operation equivalent
to an resistor element whose resistance value can be represented as
Re=1/(fCs), where f is the average switching frequency of the switch 7a
and Cs is the capacity of the capacitor 7b.

[0114]Because the switched capacitor circuit that is equivalent to a
resistor element forms a variable resistance circuit, the signal
amplification unit 7 works as a primary low-pass filter (incomplete
integration circuit) implemented by the inverting amplifier circuit. In
this case, the gain of the signal amplification unit 7 is determined by
the ratio between the feedback resistance and the input resistance.
Therefore, in the configuration described above, the gain of the
amplifier circuit can be made variable in the reverse direction of the
variation characteristic of the reference signal by configuring the
feedback resistor with the switched capacitor circuit and by changing the
equivalent resistance of this switched capacitor circuit according to the
frequency of the reference signal Vref.

[0115]The signal amplification unit 7, which uses a switched capacitor
circuit, gives a high linearity if a capacitor having no voltage
dependency is used for the capacitor. To implement a capacitor having
such characteristics on a semiconductor chip, a general two-layer
polysilicon process should be used to configure a capacitor whose
electrodes are made of polysilicon. Note that, because the non-inverting
amplifier circuit in FIG. 11B has the same configuration as that of the
inverting amplifier circuit in FIG. 11A, the description is omitted here.

[0116]Although the signal amplification unit shown in FIG. 11 has the
configuration in which the operational amplifier (OP Amp) is used, the
active circuit that configures the amplifier circuit is not limited to
the operational amplifier (OP Amp) but some other element may also be
used. FIG. 12 is a diagram showing examples of the configuration in which
a bipolar transistor or a FET is used as the active circuit. FIG. 12A
shows an example of an emitter-grounded bipolar transistor. In the
configuration shown in FIG. 12A, the gain is represented as (-R2/R1). In
the configuration of the FET shown in FIG. 12B, the gain is represented
also as (-R2/R1).

[0117]In this configuration, the resistor R2 is formed by a variable
resistance circuit and the resistance value of this variable resistance
circuit is adjusted by the frequency signal generated by
frequency-converting the reference signal. This configuration makes the
increase/decrease characteristic reversed between the reference signal
and the gain of the amplifier circuit in the same way as in the example
shown in FIG. 11, allowing the output level variations in the sensor
output to be decreased when the signal level of the reference signal
varies.

[0118]FIG. 13A is a diagram showing the configuration formed by combining
the configuration shown in FIG. 11A for stabilizing the output signal
level against variations in the reference signal and the configuration
for implementing the ratiometric characteristic. In this configuration
example, a variable resistance circuit 10c is formed as the feedback
resistor of the operational amplifier (OP Amp) 21, and a variable
resistance circuit 10d is formed as the input resistor. The resistance
value of the variable resistance circuit 10c is adjusted by the frequency
signal generated by converting the voltage of the reference signal Vref
via the frequency converter 6a. On the other hand, the resistance value
of the variable resistance circuit 10d is adjusted by the frequency
signal generated by converting the voltage of the power supply voltage
Vdd via the frequency converter 6b.

[0119]The configuration for stabilizing the output signal level against
the variations in the reference signal is configured by the variable
resistance circuit 10c, and the output signal level against the
variations in the reference signal is stabilized by the operation similar
to that described in FIG. 11A. The variable resistance circuit 10d on the
input resistor side can be configured by a switched capacitor circuit in
the same way as in the variable resistance circuit 10c on the feedback
resistor side.

[0120]The switch of the variable resistance circuit 10d is configured by a
transfer gate (transmission gate) of an MOS element, and the contact
state of the switch is configured in such a way that the contact state is
switched according to the frequency signal from the frequency converter
6b. That is, the connection state of the capacitor is switched according
to the frequency signal. The switch and the capacitor can be fabricated
in the semiconductor process and configured on the same semiconductor
chip.

[0121]One end of the capacitor of the variable resistance circuit 10d is
connected to the midpoint voltage Vm, and the other end is connected to
the fixed contact of the switch. One contact of the switch is the input
terminal of the amplifier circuit for receiving the detected signal. The
other contact of the switch is connected to the inverting input terminal
of the operational amplifier (OP Amp) 21.

[0122]When the contact of the switch is connected to the detected signal
side, the capacitor stores the voltage of the detected signal. Next, when
the switch is connected to the side of the operational amplifier (OP Amp)
21, the charge stored in the capacitor is discharged to the filter
capacitor via the operational amplifier (OP Amp) 21.

[0123]In this way, the connection state of the capacitor is switched by
switching the switch of the variable resistance circuit 10d between the
detected signal side and the side of the operational amplifier (OP Amp)
21 according to the frequency signal generated by the frequency converter
6b.

[0124]The high-speed switching operation of the switch described above
causes the switched capacitor circuit 10d to perform the operation
equivalent to a resistor element whose resistance value is represented by
the inverse number of the product of the average switching frequency f of
the switch and the capacity C of the capacitor. Because the frequency
converter 6b outputs the frequency signal f according to the voltage of
the power supply voltage Vdd, the resistance value of the variable
resistance circuit 10d is inversely proportional to the power supply
voltage Vdd. Because the gain of the operational amplifier (OP Amp) 21 is
proportional to (feedback resistance/input resistance), the gain is
eventually proportional to the power supply voltage Vdd.

[0125]The non-inverting amplifier circuit in FIG. 13B can be configured by
connecting a variable resistance circuit 10e, instead of the resistor in
FIG. 11B, to the negative input of the non-inverting amplifier circuit.
The variable resistance circuit 10e receives the frequency-converted
output from the frequency converter 6b to change the resistance value in
the reverse direction of the power supply voltage Vdd.

[0126]As in FIG. 11B, a variable resistance circuit 10b receives the
frequency-converted output from the frequency converter 6a to change the
resistance value according to the voltage Vref of the reference signal.

[0127]Next, with reference to FIGS. 14 and 15, the following describes a
second embodiment in which the resistance value is selected and switched
according to the voltage of the reference signal or the driving signal.

[0128]FIG. 14 is a diagram showing an example of the configuration in
which the feedback resistor of the operational amplifier (OP Amp) is
formed by a variable resistance circuit and the resistance value of this
variable resistance circuit is made variable by selecting the selection
signal generated by the comparison circuit by comparing the divided
voltage of the reference signal Vref and the power supply voltage Vdd. In
FIG. 14, the circuit is simplified for easy understanding.

[0129]In this simplified configuration, the voltage of the reference
signal Vref is divided by a voltage dividing resistor 42 to form stepwise
divided-voltages V1 and V2, and the divided voltages V1 and V2 are input
to one of the input ends of comparison circuits 51 and 52, one for each,
respectively. The divided voltage V0 of the power supply voltage Vdd is
input to the other input terminal of the comparison circuits 51 and 52.

[0130]A feedback resistor 16 of an operational amplifier (OP Amp) 21 of
the amplifier circuit is formed by a variable resistance circuit, and the
resistance value of this variable resistance circuit is selected based on
the comparison result of the comparison circuits 51 and 52 described
above.

[0131]A selector 53 exclusively controls the connection of switches S1,
S2, and S3 based on the comparison result of the comparison circuits 51
and 52. The switch S1 is selected if V0<V1<V2, the switch S2 is
selected if V1<V0<V2, and the switch S3 is selected if
V1<V2<V0.

[0132]When the switch S1 is on, the value of the feedback resistor 16
becomes low and the gain of the amplifier circuit becomes low. When the
switch S2 is on, the value of the feedback resistor 16 becomes
intermediate and the gain of the amplifier circuit becomes intermediate.
When the switch S3 is on, the value of the feedback resistor 16 becomes
high and the gain of the amplifier circuit becomes high.

[0133]Next, with reference to FIG. 15, the following describes the
relation between the direction into which the reference signal is
increased or decreased and the direction into which the gain selected by
the switch is increased or decreased.

[0134]FIG. 15A shows the relation between the reference signal Vref and
the divided voltages V1-V2 generated by dividing the voltage of the
reference signal Vref and the relation between the reference signal Vref
and the power supply voltage Vdd used for comparison by the comparison
circuit. Especially, the figure shows the states of V1 and V2 when the
reference signal Vref varies. The voltage used for comparison by the
comparison circuit is the divided voltage V0 generated by dividing the
power supply voltage Vdd.

[0135]The comparison circuits 51 and 52 compare the divided voltages V1
and V2 with the power supply voltage Vdd that is used as the threshold.
For example, when the reference signal Vref varies and its level becomes
low, the operation is performed as follows. When the reference signal
Vref varies and its level becomes low, its divided voltages V1 and V2
(V1<V2) also become low. In the range in which the higher divided
voltage V2 is lower than V0 (V2<V0), the selector 53 selects the
switch S3 and selects a high value for the feedback resistance. As a
result, the gain of the amplifier circuit becomes high.

[0136]When the reference signal Vref varies and its level becomes higher
than the low level, the operation is performed as follows. When the
reference signal Vref varies and its level becomes intermediate and, in
the range V1<V0<V2 in which the divided voltage V1 is lower than V0
and the divided voltage V2 is higher than V0, the selector 53 selects the
switch S2 and selects an intermediate value for the feedback resistance.
As a result, the gain of the amplifier circuit becomes intermediate.

[0137]When the reference signal Vref varies and its level becomes high,
the operation is performed as follows. When the reference signal Vref
varies and its level becomes high, the divided voltages V1 and V2
(V1<V2) also become high. In the range V0<V1 in which the lower
divided voltage V1 is higher than V0, the selector 53 selects the switch
S1 and selects a low value for the feedback resistance. As a result, the
gain of the amplifier circuit becomes low.

[0138]So, the increase/decrease in the variations of the reference signal
and the increase/decrease in the gain are in the reverse direction.

[0139]Next, with reference to FIG. 15B, the following describes the
relation between the direction in which the power supply voltage is
increased or decreased and the direction in which the gain selected by
the switch is increased or decreased.

[0140]FIG. 15B shows the relation between the reference signal Vref and
the divided voltages V1-V2 generated by dividing the voltage of the
reference signal Vref and the relation between the reference signal Vref
and the power supply voltage Vdd used in comparison by the comparison
circuit. Especially, the figure shows V0 when the power supply voltage
Vdd varies.

[0141]The comparison circuits 51 and 52 compare the divided voltages V1
and V2 with the power supply voltage Vdd that is used as the threshold.
When the power supply voltage Vdd is low and V0 is lower than V1
(V0<V1), the selector 53 selects the switch S1 and the gain of the
amplifier circuit becomes low. When the power supply voltage Vdd is
intermediate and the V0 is higher than V1 but is lower than V2
(V1<V0<V2), the selector 53 selects the switch S2 and the gain of
the amplifier circuit becomes intermediate. When the power supply voltage
Vdd is higher than V2 (V2<V0), the selector 53 selects the switch S3
and the gain of the amplifier circuit becomes high.

[0142]So, the increase/decrease in the variations of the power supply
voltage and the increase/decrease in the gain are in the same direction.

[0143]The ratio of the change in the gain of the amplifier circuit to the
change in the power supply voltage and the reference signal according to
the present invention can be determined arbitrarily by the division ratio
of the voltage-dividing resistors and the variation steps of the variable
resistor. This allows the configuration to be built so that the gain is
changed in proportion to the power supply voltage Vdd and the gain is
changed in inverse proportion to the reference signal Vref.

[0144]It is also possible to change the gain of the amplifier circuit
almost linearly by increasing the number of variation steps of the
comparison circuit and the variable resistance circuit for higher
resolution. That is, the configuration of the present invention allows
the output sensitivity of the physical quantity sensor to be kept
constant against the variations in the reference signal and, at the same
time, allows the physical quantity sensor to have the characteristic
ratiometric to a change in the power supply voltage.

[0145]Next, with reference to FIG. 16, a third embodiment will be
described in which the voltage of the reference signal or the driving
signal is converted to the current for use in changing the resistance
value.

[0146]FIG. 16 is a diagram showing an example of the configuration of an
amplifier circuit 5 that uses a voltage-current conversion circuit (OTA:
operational transconductance amplifier).

[0147]An OTA 32 constitutes the input resistor of an operational amplifier
31. The OTA 32 functions as a variable resistance circuit whose
transconductance (gm) is changed according to the current signal from a
voltage/current conversion circuit 33 that receives the reference signal
Vref. The resistance value Rin of the variable resistance circuit is the
inverse number of the transconductance (gm) and, by changing the
transconductance (gm) via the reference signal Vref, the resistance ratio
between the input resistance Rin and the feedback resistance Rf is
changed for changing the gain.

[0148]The physical quantity sensor of the present invention is applicable
to a vibration-type gyro sensor and a vibration-type acceleration sensor.